The basic objective of this research is the elucidation of the mechanism of virus induced neoplastic transformation. The system of Rous sarcoma virus, avian leukosis virus and chicken cells will be used as a model. Four major approaches will be used. First, investigations on the genetics of these viruses will be continued in order to understand the sequence of genes for various viral functions and to study the mechanism of genetic recombination and deletion of genes in the process of virus reproduction. The information concerning the maximum number of viral genomes which can be stably integrated into the cellular chromosomes will be obtained. Second, the role of endogenous virus genes in viral carcinogenesis will be studied. The effect of exogenous virus infection on the expression of the endogenous virus genes will be assessed in order to examine the mechanism of regulation of these viral genes. The studies on the viruses recovered from some avian species other than chickens will be continued. Third, a putative protein responsible for initiating various phenotypic alteration in transformation process will be identified by the use of various techniques. Finally, attempts will be made to characterize both cellular and viral structures involved in early process, namely virus attachment and virus penetration.